The present invention relates to a two-stroke internal combustion engine which is suited for use for example in a portable power working machine, and in particular, to a two-stroke internal combustion engine which is capable of rendering combustion waste gas (exhaust gas) per se to be more completely cleaned and also capable of minimizing as much as possible the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged without being utilized for the combustion.
An ordinary two-stroke gasoline engine which is conventionally used in a portable power working machine such as a chain saw or brush cutter is constructed such that an ignition plug is disposed at the head portion of the cylinder, and that an intake port, a scavenging port and an exhaust port, which are to be opened and closed by a piston, are provided so as to communicate with the cylinder bore (or provided in the inner peripheral wall of the cylinder). According to this two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the piston without undergoing a stroke which is exclusively assigned to the intake or the exhaust.
More specifically, in the ascending stroke of the piston, an air-fuel mixture consisting of a mixture comprising of air, fuel and lubricant is introduced from the intake port into the crank chamber disposed below the piston. Then, in the descending stroke of the piston, the air-fuel mixture is pre-compressed in the crank chamber producing a compressed gas mixture, which is then blown into a combustion actuating chamber which is disposed above the piston, thereby enabling waste combustion gas to be discharged from the exhaust port. In other words, the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture.
Therefore, the unburnt air-fuel mixture is more likely to be mingled into the combustion gas (exhaust gas), thus increasing the quantity of so-called blow-by or the quantity of air-fuel mixture to be discharged into air atmosphere without being utilized for combustion. Because of this, as compared with a four-stroke engine, the two-stroke internal combustion engine is not only inferior in fuel consumption but also disadvantageous in that a large amount of poisonous components such as HC (unburnt components in a fuel) and CO (incomplete combustion components in a fuel) are caused to be included into the exhaust gas. Therefore, even if the two-stroke engine is small in capacity, the influence of these poisonous components on the environmental contamination cannot be disregarded. Additionally, there are several problems as to how to cope with the regulation of exhaust gas which is expected to become increasingly severe in the future. In particular, there are many difficulties as to how to cope with the minimization of HC (total HC) in exhaust gas.
Further, since the two-stroke internal combustion engine is designed to employ a blended fuel consisting of gasoline as fuel and lubricating oil, there are possibilities that the exhaust gas would be further contaminated due to this oil component, and that this oil component would be allowed to excessively flow into the combustion actuating chamber (though it may also be called combustion chamber, actuating chamber, cylinder chamber, etc., these chambers are generically referred to as a combustion actuating chamber in the present specification), thereby sometimes inviting operating malfunctions such as engine stalls.
With a view to overcome these problems, there have been proposed various kinds of countermeasures. For example, JP Patent Publication No. 60-48609 (1985) discloses a two-stroke internal combustion engine which is provided with a scavenging passageway of a reverse scavenging system (Schnürle type scavenging system) where the combustion actuating chamber to be formed over the piston is communicated with the crank chamber. This two-stroke internal combustion engine has the cross-sectional area of the scavenging outlet port (situated on the combustion actuating chamber side) of the scavenging passageway made smaller (reducing the cross-sectional area to 60% or less) than the cross-sectional area of the scavenging inlet port (situated on the crank chamber side) to thereby enhance the flow rate of scavenging to be blown into the combustion actuating chamber, thus enhancing the scavenging efficiency and minimizing the quantity of blow-by.
Further, JP Laid-open Patent Publication (Kokai) No. 2000-179346 discloses that if the cross-sectional area of the inlet port of the scavenging passageway is approximately the same as that of the outlet port of the scavenging passageway, the air-fuel mixture (scavenging gas) that has been pushed out of the crank chamber is permitted to rapidly flow into the combustion actuating chamber from the scavenging outlet port, thus permitting a large quantity of the air-fuel mixture to be discharged together with exhaust gas to the external atmosphere. In order to prevent such a phenomenon, the publication proposes considerably contracting the cross-sectional area of the inlet port of the scavenging passageway (or scavenging introducing passageway) as compared with the cross-sectional area of the scavenging passageway. By contracting the scavenging inlet port (or scavenging introducing passageway) in this manner, it is possible to prevent the scavenging gas from rapidly flow into the combustion actuating chamber from the scavenging outlet port, so that the scavenging gas can be gradually fed to the combustion actuating chamber until the scavenging stroke is placed into the latter half stage thereof. As a result, the quantity of blow-by can be greatly minimized.
In the case of the two-stroke internal combustion engine described in JP Patent Publication No. 60-48609, however, the compression ratio (pressure) of the air-fuel mixture inside the crank chamber is caused to become lower in the latter half stage (in the vicinity of bottom dead center) of the descending stroke (scavenging stroke) as compared with the compression ratio of the air-fuel mixture up to the intermediate stage of the descending stroke so that the flow rate of the scavenging gas to be blown into the combustion actuating chamber from the scavenging outlet is also caused to decrease. Therefore, it is impossible to obtain a sufficient degree of scavenging effect as desired. Rather, scavenging gas (air-fuel mixture) is more likely to be intermingled with the exhaust gas and hence it is impossible to sufficiently minimize the quantity of blow-by.
Further, in the case of the two-stroke internal combustion engine described in JP Laid-open Patent Publication (Kokai) No. 2000-179346, since the scavenging gas is prevented from rapidly flowing into the combustion actuating chamber from the scavenging outlet port due to the effect of the contracted scavenging outlet port (scavenging introducing passageway), the flow rate of the scavenging gas flow in particular in the first half stage of the descending stroke (scavenging stroke) of the piston is caused to become lower than that of the scavenging flow which is described in JP Patent Publication No. 60-48609, so that it is impossible to expect the minimization in quantity of blow-by in such a sufficient degree as desired.
The present invention has been made in view of overcoming the aforementioned problems, and therefore an object of the present invention is to provide a two-stroke internal combustion engine which can be manufactured without necessitating the tremendous modification of the structure thereof, which can be manufactured at low cost, and which is capable of effectively suppressing the blow-by of unburnt air-fuel mixture, of minimizing as much as possible poisonous components such as HC, etc. that may be discharged into air atmosphere, and of preventing the malfunctions that may be brought about due to excessive supply of the lubricating oil into the combustion actuating chamber.